Detection and prevention of hot spots in a solar panel

ABSTRACT

An electronic module compares the output voltage of a solar panel to an expected value and controls the power demand from the solar panel such that the output voltage does not vary from the expected value by more than a predetermined value. The predetermined value may be determined by correcting a room temperature value for the temperature dependence of the photodiodes comprising the solar panel and manufacturing tolerance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to commonly-owned U.S. patent applicationSer. No. 12/061,025 submitted Apr. 2, 2008 by Kernahan et al, whichapplication is incorporated herein in its entirety.

BACKGROUND

Solar panels are expected by their makers to last at least twenty fiveyears. One of many lifetime-limiting conditions to be dealt with toenable such a long lifetime is hot spots on the panel. Hot spots maylimit lifetime by causing damage to the panel due to heat generatedand/or longer term degradation of the panel cell material due todiffusion aging. Failure modes include melting solder joints, pin holesor open circuits in a cell, and damage to the panel case. Some causes ofhot spots are manufacturing related, such as an assembly flaw,substandard materials, contamination of a solar cell, and thealways-present manufacturing variations. Though a panel may have beenmanufactured with flaws, it may well be serviceable for an extendedtime, though less than expected. Other causes are beyond the control ofthe manufacturer or installer. For example, some cells in a panel may beexposed to more or less sunlight than other cells due to partial shade,dirt or bird droppings in a localized area, temperature variationsacross a panel, and non-uniform aging of the diffusion regions from cellto cell.

The destructive effects of hot-spot heating may be circumvented throughthe use of a bypass diode. A bypass diode is connected in parallel, butwith opposite polarity, to a solar cell. Under normal operation, eachsolar cell will be forward biased and therefore the bypass diode will bereverse biased and will effectively be an open circuit. However, if asolar cell is reverse biased due to a mismatch in short-circuit currentbetween several series connected cells, then the bypass diode conducts,thereby allowing the current from the good solar cells to flow in theexternal circuit rather than forward biasing each good cell. The maximumreverse bias across the poor cell is reduced by the bypass diode toabout a single diode drop, thus limiting the current and preventinghot-spot heating.

A typical circuit model of a solar panel is shown in FIG. 1. For clarityof explanation, the example is simply two cells in series. Obviously atypical panel has many more cells in series to form a “string”, and somehave multiple strings in parallel. In the model of FIG. 1, each solarcell is modeled as a current source in parallel with a reverse-biaseddiode. The example of FIG. 1 includes a cell 102 in series with a cell104, with bypass diodes 106, 112 respectively. The current of the modelarises from the photodiodes 106, 108 when exposed to adequate light. Weconsider four cases related to solar cells that are equal and unequal inpower capacity, each case in open and short circuit configurations. In ashort circuit condition and with matched cells the voltage across boththe solar cells and the bypass diodes is zero; the bypass diodes have noeffect. When open circuit (also with matched cells) the short currentfrom each cell forward biases the cell. The bypass diodes are reversebiased, and again, have no effect on the circuit.

Assume now that cell 104 is shaded, thus has less power providingcapacity than that of cell 102. For the short circuit condition, somecurrent flows from cell 102, forward biasing the cell 102. The bypassdiode 110 is again reverse biased and has no effect. The voltage of thegood cell 102 forward biases the bypass diode 112 of the weak cell 104,causing it to conduct current. The shaded cell 104 itself is reversebiased with approximately a diode drop of about −0.5 volts. For thefourth condition, that is a weak cell 104 and an open circuit, theshaded cell 104 has a reduced voltage. The bypass diodes 110, 112 arereverse biased and have no effect.

In practice, however, one bypass diode per solar cell is generally tooexpensive and instead bypass diodes are usually placed across groups ofsolar cells. The voltage across the shaded or low current solar cell isequal to the forward bias voltage of the other series cells which sharethe same bypass diode plus the voltage of the bypass diode. The voltageacross the unshaded solar cells depends on the degree of shading on thelow current cell. For example, if the cell is completely shaded, thenthe unshaded solar cells will be forward biased by their short circuitcurrent and the voltage will be about 0.6V. If the poor cell is onlypartially shaded, the some of the current from the good cells can flowthrough the circuit, and the remainder is used to forward bias eachsolar cell junction, causing a lower forward bias voltage across eachcell. The maximum power dissipation in the shaded cell is approximatelyequal to the generating capability of all cells in the group. Themaximum group size per diode, without causing damage, is about 15cells/bypass diode, for silicon cells. For a normal 36 cell module,therefore, 2 bypass diodes are used to ensure the module will not bevulnerable to “hot-spot” damage.

Consider now a typical solar panel configuration and response to partialshading. A set of 25 modules connected in series form a nominal Vmpp of467.5 V at 11.23 A or 5,250 W. Assume each module is constructed ofthree strings of 38 cells (mpp @ 492 mV, 3.743 A) each and the topmiddle and bottom of each string are connected. Between the middle oftop and middle to bottom are bypass diodes (Vf 410 mV). If one cellbecame shaded or soiled to the extent that it's current dropped by 374mA or more (10%) then two candidate operating points would be found byan MPPT scan for the string:

-   -   Approximately 467.5V @ 10.853 A or 5,075 W or    -   Approximately 457.7V @ 11.230 A or 5,140 W

Since the portion of the module with the shaded cell only produce 10.853A, its bypass diode is forced into conduction forcing the bypass diode's410 mV and the 9.350 V of the 19 bypassed cells to be subtracted fromthat modules voltage (total loss of 9.760V from the string of modules).Within the bypassed 19 cells the sum of the voltage across the good 18cells plus the voltage across the shaded cell must equal −410 mV (thevoltage across the bypass diode) at the current of the shaded cell(because all 19 cells are in series).

The solution is approximately 8.856V across the 18 good cells and−9.266V across the shaded cell @3.369 A or 31.2 w of power dissipationin the shaded cell. Note that a similar situation exists with the othertwo sets of 19 cells because they too are forced to sum to the −410 mVof the bypass diode.

The bypass diode has the difference of module string current minus thebypassed sections. The module is producing 97.026 W for a loss of 54%and dissipating an additional 100 w as heat. A string monitoring means,for example an ADC, would record a 10V drop in nominal Vmp for thestring. A technician dispatched to investigate would find a moduleoperating at 9V when he expected 18V, no change in power when he cast ashadow across half of the module and that some cells in the module wereabnormally hot (all standard trouble shooting observations). Thetechnician may conclude that the module is below the 80% limit andassert that it has failed. However at the factory, this module wouldflash test as only 3.4% below nominal at 18.7V and 10.853 A or 203 w,although it would show a current step of 374 mA (3.3%) at about 8.940V.

The result of the reversal of one or more cells varies for differingsolar cell technologies. For cells of a mono-crystalline type, there maybe no lasting damage but a loss of efficiency. For cells of a thin-filmconstruction, reversal of a voltage on a given cell is immediatelycatastrophic. As is seen, then, bypass diodes are a necessary andeffective method for diminishing hot spots caused by partial shading orother causes for a weak cell. However, looking to FIG. 2, we see thatthe strings 202, 204, 206, 208, 210, 212 have an interconnect ofconductors of a certain size which we will call size “X”. If the bypassdiodes 212, 222 conduct, they can carry as much as 3× the current of oneof the strings, therefore the conductor for each bypass diode isnormally sized as 3× that of a single string conductor. The size of thebypass diode interconnect 230, 232 then, adds significant area to theminimum area for constructing a solar panel.

What is needed is a means for avoiding hot spots without bypass diodesand their attendant area increase of a solar panel.

SUMMARY

The present invention avoids the condition of a hot spot without the useof an efficiency-lowering protection diode. The method of the presentinvention assumes an apparatus is used to control the operatingconditions of the panel, wherein the apparatus includes means formeasuring the total voltage across the strings and means for changingthe operating conditions of the panel. Bypass diodes are not needed norused, saving the area required for interconnect as typical with theprior art. In the present invention, the instant voltage is compared tothe expected voltage for a measured operating temperature. If thevoltage is less than expected by more than a certain amount, the power(current) demanded from the panel is reduced such that the voltage isless than a diode drop of the expected voltage, thereby avoiding a hotspot. With hot spots, that is reverse biasing of a weak cell, avoided,bypass diodes are not needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical model of a solar panel.

FIG. 2 is an example physical layout of a typical solar panel,specifically related to the area needed for interconnect.

FIG. 3 is a graph relating the output voltage of a solar panel to thetemperature of the solar panel.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Grid AC power provided to a premises by an outside source, typically autility company. PV Photovoltaic panel; another term for thecommonly-used “solar panel” cps Abbreviation for “cycles per second”;the frequency of an AC power supply AC Abbreviation for “alternatingcurrent”, though one may also view it as “alternating voltage” in thatthe polarity of the voltage provided alternates. DC Abbreviation for“direct current”; electrical power that is always provided in a givenpolarity. The voltage of the power source may or may not be fixed. FETField effect transistor PAM Pulse Amplitude Modulation. a form of signalmodulation where the message information is encoded in the amplitude ofa series of signal pulses. PCM Pulse Code Modulation. a digitalrepresentation of an analog signal where the magnitude of the signal issampled regularly at uniform intervals, then quantized to a series ofsymbols in a digital (usually binary) code. MPPT Maximum Power Point; acondition wherein a power source is operated at its maximum power outputcondition. In solar panels, controlling devices may frequently trydiffering operating conditions to determine the maximum power point forthe instant conditions.

Definition of Some Terms:

Array An electronic module for controlling the operation of a Convertersolar panel, disclosed in more detail in the U.S. patent applicationSer. No. 12/061,025.

According to the present invention, a solar panel is controlled by anelectronic module, the module including means for measuring thetemperature of the panel cells, the voltage across the panel, and forcontrolling the power (current) provided by the panel. A solar panel maybe expected to provide a certain output voltage under good operatingconditions, as determined by specification, characterization data, or bythe experience derived by accumulating performance data over time. Thecurrent available is a function of the intensity of sunlight incidentupon the panel, and the voltage a function of the temperature of thecells, assuming otherwise normal conditions for the cells. As describedhereinbefore, a weak cell, due to damage, deterioration, soil, or simplypartial shading of the panel, will not provide the same power as willthe other, unaffected cells. Because all cells in a string areelectrically in series, the current must be in common. Therefore theonly way the weak cell can adjust for the instant lower power capacityis by a lower voltage for that cell. Again because the cells areelectrically connected in series, the voltage across the string will bethe sum of the voltages of all the cells in the string. Obviously, then,when a cell in the string loses some voltage, the whole string does aswell.

An electronic module typically tests a panel periodically, for exampleonce per hour, to determine the maximum power point (MPPT) operatingcondition. This is accomplished by varying the current demanded from apanel, measuring the voltage across the panel, then determining thepower for that condition as the product of voltage times current. Byvarying across a certain range of currents, a peak power point may befound. In the prior art, such MPPT testing is done without regard towhether the condition selected may drive a weak cell in a string into aforward bias condition, thereby causing the bypass diodes to be forwardbiased, as described hereinbefore. According to the present invention,the electronic module first determines the temperature of the solarpanel cells, determines expected panel voltage for the temperaturefound, and does not allow the current to cause the voltage to drop morethan a predetermined amount below the expected voltage. For example, inone embodiment the maximum value below MPP to be allowed is:

RT_(MPP)−tolerance−degredation(temp)

wherein RT_(MPP) is the maximum power point condition for roomtemperature, “tolerance” is a value provided by the solar panelmanufacturer, and degredation(temp) is the diode drop value that resultsfrom increasing temperature, for example −2.1 mv/degree C. for a siliconsolar cell. Of course these values will be different for other solarcell chemistries.

The result is that, if there were in fact bypass diodes the bypassdiodes would never be forward biased, therefore the diodes are notneeded and a solar panel designed for an electronic module according tothe present invention is made without bypass diodes, thereby saving thearea that would be required for the interconnect of the bypass diodes.

Consider an example, wherein a set of twenty-five modules are connectedin parallel form a total array of 5,250 W. Each panel is controlled byan individual electronic module connected to the panel, for example anArray Converter as disclosed in the '025 application, wherein theelectronic module includes means for measuring the voltage across thestrings and for controlling the current demanded from its associatedmodule. Assume each module is constructed of one string of 114 cells(mpp @492 mV, 3.743 A). If one cell became shaded or soiled to theextent that it's current dropped by 374 mA (10%) then the power for thatmodule only would be reduced by 10%. The array converter will only bepermitted the MPP solution of approximately 56.088V*3.369 or 189 w (10%loss). This is because any solution lower than 90% (a programmablelimit) of nominal Vmp at the measured temperature would not be allowedas an MPPT solution. This ensures that an Array Converter would notreverse a cell by more than 5.6V (half the amount of the bypass diodeapproach) even during an MPPT search.

Since the other 24 array converter modules would remain unaffected, thetotal power is 5,228 w vs 5,140 w for the string inverter case. Thesingle module with the single shaded cell does not dissipate anyadditional power.

1. A method for controlling the operation of a solar panel by anelectronic module, wherein the electronic module includes means formeasuring a value of the temperature of the solar panel and means formeasuring a value of the voltage across the solar panel and means forconfiguring the electronic module to demand more or less current fromthe solar panel, comprising: determining a value of the temperature ofthe solar panel; determining an expected output voltage of the solarpanel as a function of the value of the temperature; determining aninstant value of the output voltage of the solar panel; comparing theinstant value of the output voltage of the solar panel to the expectedvalue of the output voltage of the solar panel; and configuring theelectronic module so that a value of current drawn from the solar panelprevents the instant value of the output voltage from exceeding anegative difference value of the expected output voltage.
 2. The methodaccording to claim 1, wherein the expected output voltage is determinedfrom a specification of the solar panel.
 3. The method according toclaim 1, wherein the expected output voltage is determined fromcharacteristic data.
 4. The method according to claim 1, wherein theexpected output voltage is determined according to weather data.
 5. Themethod according to claim 1, wherein the expected output voltage isdetermined by calculating a rolling average over time.
 6. The methodaccording to claim 1, wherein the expected output voltage is determinedby comparing the output voltage of a given solar panel to the outputvoltage of other solar panels in a common system.
 7. The methodaccording to claim 1 wherein the negative difference value ispredetermined by a manufacturer of the solar panel.
 8. The methodaccording to claim 1, wherein the negative difference value isdetermined by adjusting a maximum power point voltage condition bysubtracting a tolerance value and by subtracting a temperaturecorrection factor characteristic of the solar panel.